How to exact copy paint weights from one object to another
I know this is 4 years old post but I've found a simpler solution with blender 2.82. So what I do is I copy the mesh that has three original vertex groups and arrange in the space where the 2nd mesh is meant to be at. Then rename the 2nd mesh's all vertex groups to different so it can retain its weight paint from original mesh. I used armature human rig default.